Assessment of specificity of eight chemical inhibitors using cDNA-expressed cytochromes P450

Abstract

1. The selectivity of eight chemical inhibitors has been extensively evaluated with 10 cDNA-expressed human cytochrome P450 isoforms (CYP). The results indicate that sulphaphenazole, quinidine and α-naphthoflavone are selective inhibitors of CYP2C9 (IC₅₀ = 5 0.5-0.7 μM), CYP2D6 (0.3-0.4 μM) and CYP1A (0.05-5 μM) respectively on the basis of the IC₅₀, which are much lower than those of other P450 isoforms (> 10-fold). 2. Ketoconazole exhibited potent inhibition of both CYP3A4-catalysed metabolism of phenanthrene, testosterone, diazepam (IC₅₀ = 0.03-0.5 μM) and CYP1A1-catalysed deethylation of 7-ethoxycoumarin (0.33 μM). The selectivity of ketoconazole for other P450s was highly related to the concentration used. 3. Diethyldithiocarbamate, orphenadrine and furafylline were shown separately to be less selective inhibitors of CYP2E1, CYP2B6 and CYP1A isoforms by a broad range of IC₅₀ that overlap those observed with other P450 isoforms. 4. Furafylline, quinidine and α-naphthoflavone activated CYP3A4-catalysed phenanthrene metabolism by 1.7-, 2- and 15-fold respectively. 5. The selectivity of orphenadrine and ketoconazole was further examined by using inhibitory monoclonal antibodies (MAb). Inhibitory MAb specific for the individual P450 isoforms may be of greater value than chemical inhibitors.